Cancer of the prostate is the most commonly diagnosed cancer in men and is the second most common cause of cancer death (Carter et al, 1990; Armbruster et al, 1993). If detected at an early stage, prostate cancer is potentially curable. However, a majority of cases are diagnosed at later stages when metastasis of the primary tumor has already occurred (Wang et al, 1982). Even early diagnosis is problematic because not all individuals who test positive in these screens develop cancer. Present treatment for prostate cancer includes radical prostatectomy, radiation therapy, or hormonal therapy. No systemic therapy has clearly improved survival in cases of hormone refractory disease. With surgical intervention, complete eradication of the tumor is not always achieved and the observed re-occurrence of the cancer (12-68%) is dependent upon the initial clinical tumor stage (Zietman et al, 1993). Thus, alternative methods of treatment including prophylaxis or prevention are desirable.
Prostate specific antigen (PSA) is a 240 amino acid member of the glandular kallikrein gene family. (Wang et al, 1982; Wang et al, 1979; Bilhartz et al, 1991). PSA is a serine protease, produced by normal prostatic tissue, and secreted exclusively by the epithelial cells lining prostatic acini and ducts (Wang et al, 1982; Wang et al, 1979; Lilja et al, 1993). Prostatic specific antigen can be detected at low levels in the sera of healthy males without clinical evidence of prostate cancer. However, during neoplastic states, circulating levels of this antigen increase dramatically, correlating with the clinical stage of the disease (Schellhammer et al, 1993; Huang et al, 1993; Kleer et al, 1993; Oesterling et al, 1991). Prostatic specific antigen is now the most widely used marker for prostate cancer. The tissue specificity of this antigen makes PSA a potential target antigen for active specific immunotherapy (Armbruster et al, 1993; Brawer et al, 1989), especially in patients who have undergone a radical prostatectomy in which the only PSA expressing tissue in the body should be in metastatic deposits. Recent studies using in-vitro immunization have shown the generation of CD4 and CD8 cells specific for PSA (Peace et al, 1994; Correale et al, 1995). However, although weak natural killer cell responses have been occasionally documented in prostate cancer patients (Choe et al, 1987), attempts to generate an in vivo immune response have met with limited success. For example, several attempts to actively immunize patients with prostate adenocarcinoma cells admixed with Bacillus Calmette-Gurein (BCG) have shown little or no therapeutic benefit (Donovan et al, 1990). The ability to elicit an immune response as a result of exposure to PSA in vivo would be extremely useful.
Vaccinia virus has been used in the world-wide eradication of smallpox. This virus has been shown to express a wide range of inserted genes, including several tumor associated genes such as p97, HER2/neu, p53 and ETA (Paoletti et al, 1993). Other pox viruses that have been suggested as useful for expression of multiple genes include avipox such as fowl pox. Cytokines expressed by recombinant vaccinia virus include IL-1, IL-2, IL-5, IL-6, TNF-α and IFN-γ (Paoletti et al, 1993). Recombinant pox viruses, for example vaccinia viruses, are being considered for use in therapy of cancer because it has been shown in animal models that the co-presentation of a weak immunogen with the highly immunogenic poxvirus proteins can elicit a strong immune response against the inserted gene product (Kaufman et al, 1991, Paoletti et al, 1993; Kantor et al, 1992a; Kantor et al, 1992b; Irvine et al, 1993; Moss et al, 1993). A recombinant vaccinia virus containing the human carcinoembryonic antigen gene has just completed phase 1 clinical trials in carcinoma patients with no evidence of toxicity other than that observed with the wild type smallpox vaccine (Kantor et al, 1992b).
Currently, models for the evaluation of prostate therapeutics include the canine (McEntee et al, 1987) and the Dunning rat (Isaacs et al, 1986); neither of these models, however, are practical for the study of PSA-recombinant vaccines due to the very low homology of rat and canine PSA to human PSA (Karr et al, 1995; Schroder et al, 1982). In contrast, the prostate gland of the rhesus monkey is structurally and functionally similar to the human prostate (Wakui et al, 1992). At the molecular level there is 94% homology between either the amino acid or nucleic acid sequences of rhesus PSA (Gauther et al, 1993) and those sequences of human prostate specific antigen (Karr et al, 1995; Lundwall et al, 1987). Thus, human PSA is essentially an autoantigen in the rhesus monkey. Accordingly, the rhesus monkey can serve as a model for autologous anti-PSA immune reactions.
Since PSA shares extensive homology with members of the kallikrein gene family which are expressed in normal tissue, it is important to use minimal epitope peptides to avoid unwarranted cross reactivity. These epitopes have been selected for their divergence with members of the kallikrein gene family.
Studies disclosed in U.S. Ser. No. 08/500,306 have shown that two PSA epitope peptides (PSA-1 and PSA-3), 10-mers selected to conform to human HLA class 1-A2 motifs, can elicit CTL responses in both normal donors and patients with prostate cancer. (Correale et al, 1995) The present invention discloses the advantage of PSA-oligo-epitope peptides comprising more than one PSA epitope peptide in generating PSA specific cellular immune responses.